The latex aggregation method has long been used for detecting a target substance in a sample. In the latex aggregation method, in order to detect an antigen present in liquid such as a biological sample, the liquid and latex carrying an antibody or a fragment thereof that specifically binds to the target antigen are mixed, and the degree of latex aggregation is measured to detect or quantify the antigen (e.g., Japanese Published Examined Patent Application No. 558-11575, hereinafter referred to as Patent Document 1).
According to the latex aggregation method, aggregation of latex is facilitated by an antigen, which is added as a sample and cross-links a plurality of latex-bound antibodies. This simple procedure allows for convenient and rapid detection of an antigen. However, when the amount of the antigen is small, since it is difficult to generate cross-linking, a sufficient amount of latex cannot aggregate. Therefore, it has been difficult to detect a small amount of antigen.
Thus, methods utilizing an enzyme-substrate reaction, such as ELISA and CLEIA, are widely used. In these methods, for example, a primary antibody that binds specifically to an antigen is bound to an antigen, and a secondary antibody having an enzyme is bound to this primary antibody. Then, an enzyme substrate is added and the reactivity of a reaction catalyzed by the enzyme is measured to detect or quantify an antigen.
According to these methods, by using a luminescent reagent as a substrate, for example, the high detectability of a luminous reaction after adding the substrate allows detection of an extremely small amount of antigen.
However, the methods utilizing an enzyme-substrate reaction require a number of special reagents such as a secondary antibody and luminescent reagent, which make the operating cost high. Moreover, since the measuring process must be completed in an extremely short period of time to avoid color degradation (bleaching phenomenon) of the luminescent reagent, insufficiently accurate results are likely.
Meanwhile, these methods consist of a plurality of steps that make the operation complex, such as incubation of the specimen and each reagent, cleaning of the system, and detection of the luminous reaction. Each of these steps takes considerable time, and therefore these methods are not suitable for large-scale processing.